1. Field of the Invention
The present invention relates to composite structures comprising a layer of a gas filled cellular material, a layer of an ABS sheet having a substantial number of rubber particles in the range of from about 4 to about 10 microns and a layer of a thermoplastic polymer such as ABS.
2. Description of the Prior Art
Composite structures comprising a cellular insulation material bonded to a thermoplastic sheet are commonly prepared by foaming-in-place techniques. The resulting composite structure is used in a wide variety of applications to provide thermal insulation, acoustical insulation and in certain applications buoyancy. Examples of these applications include refrigerator construction, boat hulls, panels for recreation vehicles, door panels, wall panels, headliners for sea and air vehicles, etc. Generally the thermoplastic sheet is presented as the exterior surface of the composite while the cellular insulation material is hidden from view in the end use application.
In most of these composite structures for refrigeration use, gas filled polyurethane foam is used as the cellular insulation material and acrylonitrile-butadiene-styrene (ABS) polymer or rubber modified polystyrene is used as the thermoplastic sheet material. However, other thermoplastic polymeric sheet materials can be used as is discussed in greater detail below. The gas found in the cells of the insulation material is usually an aliphatic or cycloaliphatic chlorofluorocarbon (CFC's). These gases are present in the cells of the cellular insulation material as residual blowing agents. Alternately, they are incorporated into the cells by design in order to improve the thermal insulation properties of the cellular material. Examples of these fluorochlorocarbons include trichlorofluoromethane, dichlorodifluoromethane, 1,1,2-trichloro-1,2,2-trifluoroethane, octafluorocyclobutane, and mixtures thereof. Recently, the above fluorocarbons are being replaced with hydrochlorofluorocarbons (HCFC's) such as dichlorotrifluoroethane (HCFC 123), dichlorofluoroethane (HCFC 141b), chlorodifluoromethane (HCFC 22), chlorodifluoroethane (HCFC 142b) and tetrafluoroethane (HCFC 134a).
Fully halogenated chlorofluorocarbons [such as trichlorofluoromethane (CFCl.sub.3) or chlorofluorocarbon (CFC)-11] are extremely stable and break down only when they have reached the ozone layer, where ultraviolet radiation breaks them down causing the release of chlorine radicals. The chlorine radicals are believed to react with ozone to form oxygen thereby reducing the ozone in the stratosphere. HCFC's are CFC's containing at least one hydrogen atom. These appear as a desirable replacement for the CFC's in that they break down more readily in the troposphere or lower atmosphere. Therefore, they have less ozone depletion potential. Yet, HCFC's are more aggressive than CFC's towards high impact polystyrene (HIPS) and ABS resins.
Accordingly, the composites described above suffer from a disadvantage in that the thermoplastic sheet becomes less impact resistant and more susceptible to cracking when bonded to an HCFC filled foam. The loss of impact resistance and the tendency to crack is reduced by the use of an elastomeric layer between the gas filled foam and the ABS as taught in U.S. Pat. No. 3,563,845 to J. Stevens. In U.S. Pat. No. 3,565,746, the same patentee teaches the use of three-layer systems comprised of an organic, rigid polymeric solid layer, such as a graft copolymer blend of a monovinyl aromatic compound, an alpha-electronegatively substituted ethene, and a conjugated alkadiene; and two different layers of gas filled cellular material. The gas-filled cellular material layers differ in density and compressive modulus in order to achieve improved impact resistance. Other methods used in the art to preserve the impact strength of the ABS layer include the use of a high nitrile resin layer, a wax layer or a layer of polyethylene between the cellular material and the ABS.
The methods described above provide some improvement in the retention of impact resistance of the thermoplastic sheet laminated to the gas filled cellular material. However, the impact resistance of the thermoplastic sheet may still deteriorate due to the accelerated aging of the layers which are interposed between the cellular material and the thermoplastic sheet. In addition, the HCFC in the cellular material may permeate the intermediate layers and attack the thermoplastic sheet. In either event the result is a decrease in impact strength of the thermoplastic sheet in the composite structure.
A need exists in the art for improved composite structures wherein the thermoplastic sheet component is protected against the halogenated carbons and hydrocarbons and especially the HCFC's found in the cellular insulation material bonded to the thermoplastic sheet while maintaining the rigidity and impact strength of the composite structure.
A need also exists for improved composite structures for use as refrigerator liners which will resist attack by the HCFC's.